I got the urge to build a tube guitar amp. My first exposure to tubes was a Sano [not Sanyo] accordion amp that I refurbished and modified for guitars. I learned to play on that amp and foolishly sold it when I moved into an apartment. That was fifteen years ago and my guitar has been sitting unused in the back of the closet ever since.

The plan is to build the amp mostly from parts laying around. I thought it would be fun to design it around oddball [$1] tubes instead of the standard 12A*7 found in most amps.

I have no trouble with electrical and metalworking but don't have the tools [or skills] for woodworking. I have been searching craigslist, thrift shops, and garage sales for a speaker cabinet or cheap transistor combo amp that I could repurpose. Then, this morning, I was driving through a neighborhood and saw a speaker cabinet sitting on the curb. I knocked on the door and asked the lady if she was getting rid of it and she said, "Yes, take it."

It's missing the speakers but the back is still on it and the silver grill cloth is flawless. The only ugly spot is from someone using it as a sawhorse. They cut into the top of it half a dozen times with a circular saw.

It's made for two 12" speakers which is a lot bigger than I was planning to build. Finding speakers to fill the holes will be the biggest expense. On the plus side, there should be no trouble fitting a long reverb tank in there.

Nice box... Flipper can clue you in on dollar tubes, he's built several with those tube specials...

As far as 12AX7, it's often overlooked the 6/12AV6 triode is same as one section of the 12AX7, includes 3/4AV6 as well... Pair of 3AV6 with heaters in series, bam instant 12AX7(well it'll draw 300ma additional heater current)... Also several variants of the slightly lower gain 6AT6 incl 6AQ6, 6CN7, 6T8 etc... If you wanna go big, there are Compactrons with multiple triodes that equal 12AX/AT/AU7.. The 4/5HA7 are one section each of 12AX7 & 12AU7... With a 6v transformer it's easy to install a dropping resistor at the tube and use the 5v variants...

How much power are you trying to get and just how cheap, overall, are you shooting for? Dollar days tubes might not get you much by the time you figure the iron and, as you pointed out, speakers. And then just the 'nitty gritty' mounts up.

Can't say I'd build it that way nowadays but some items of note. The OPT was a (honker) 18CVT power transformer out of a 12 Volt linear power supply that self destructed. The 6DT8 is a cheap substitute for 12AT7 (I had two spare 12AT7 as a cheap, albeit lower gain, alternative for the 12AX7). I had two 6CW5 (got a box of 'tubes' for free) and it's capable of 25 Watt at 250 volt, but requires a lower screen which is a pain. But they're rather cheap too. I made the amp switchable between cathode and fixed bias which meant it had to be regulated as well. I used solid state for that and also a MOSFET follower for the tone stack to save a tube. The power transformer was a junker which required some heavy handed dropping resistors to make it all 'fit'.

Just some examples of how one thing leads to another so 'cheap' at one end often leads to 'more cost' at the other.

Despite being a frankenamp (are you going to use THAT?) my nephew's recording studio said it was the best sounding amp they'd ever seen. I have no idea why but perhaps it was the 'poor' HF response of the power transformer OPT simply not able to reproduce high end harmonics. Or they could have just been blowing smoke up his read end but I was all puffy chest for a while

I'm thinking of keeping it under 20 watts and overall cost isn't a big concern as long as the individual parts are relatively cheap. A couple $20 purchases every couple weeks flies right under the wife's radar, but any single thing over $50 sends up red flags. I try not to keep any records of how much I actually spend on projects for fear it may incriminate me.

Got another big piece of the puzzle today, a couple alnico speakers from an organ. Only $30 for the pair but it ended up being a two and a half hour trek. They were only about 20 miles away but Portland traffic is a nightmare.

I'm thinking of keeping it under 20 watts and overall cost isn't a big concern as long as the individual parts are relatively cheap. A couple $20 purchases every couple weeks flies right under the wife's radar, but any single thing over $50 sends up red flags. I try not to keep any records of how much I actually spend on projects for fear it may incriminate me.

Got another big piece of the puzzle today, a couple alnico speakers from an organ. Only $30 for the pair but it ended up being a two and a half hour trek. They were only about 20 miles away but Portland traffic is a nightmare...

I see. Clever plan to avoid, uh, repercussions.

You might next need to decide on an OPT. Depending on your choice that might dictate or limit your pick of output tubes. The 12AB5 is a low cost 6V6 in a 9 pin socket and the 6HB6 is sort of a 'super' 6BQ5. Basically same plate curves but almost twice the gm. Both are 3 bucks. The 6CW5 I used is only 5 bucks but it's not easy to find a 3k PP OPT.

Speaking of compactrons, there are number of 'dollar days' duals with an audio pentode (similar to 6V6) paired with a dual control pentode that was intended as an FM detector but if it's biased right could probably be used as a 'normal' pentode or triode mode. The 6AD10 is one.

Hmm... a dual control pentode might make for some interesting effects in a guitar amp.

Might be interesting to give it a shot. To make it a 'normal' pentode G3 needs to be about 3V positive. Less and the plate knee gets mushy with increased distortion and if you take it negative the knee vanishes entirely.

You might next need to decide on an OPT. Depending on your choice that might dictate or limit your pick of output tubes. The 12AB5 is a low cost 6V6 in a 9 pin socket and the 6HB6 is sort of a 'super' 6BQ5. Basically same plate curves but almost twice the gm. Both are 3 bucks. The 6CW5 I used is only 5 bucks but it's not easy to find a 3k PP OPT.

I have an old junker Bogen CHB100 PA amp that I picked up at a flea market 20 years ago. It's missing a lot of stuff but both transformers are there. I just measured the output transformer; feeding 1 VAC into the 4 ohm tap produced 23.48 volts between the plate leads, 1 VAC into the 8 ohm tap was 16.89 volts, 1 VAC into the 16 ohm tap was 11.92 volts. So, if I'm doing the math right, the impedance is roughly 2250 ohms. Any suggestions for output tubes that will work with this OPT?

You might next need to decide on an OPT. Depending on your choice that might dictate or limit your pick of output tubes. The 12AB5 is a low cost 6V6 in a 9 pin socket and the 6HB6 is sort of a 'super' 6BQ5. Basically same plate curves but almost twice the gm. Both are 3 bucks. The 6CW5 I used is only 5 bucks but it's not easy to find a 3k PP OPT.

I have an old junker Bogen CHB100 PA amp that I picked up at a flea market 20 years ago. It's missing a lot of stuff but both transformers are there. I just measured the output transformer; feeding 1 VAC into the 4 ohm tap produced 23.48 volts between the plate leads, 1 VAC into the 8 ohm tap was 16.89 volts, 1 VAC into the 16 ohm tap was 11.92 volts. So, if I'm doing the math right, the impedance is roughly 2250 ohms. Any suggestions for output tubes that will work with this OPT?

Zowie! That's a 100 Watt OPT originally used with quad 7868s. It should be something like 3300 Ohm or maybe 3000 Ohm plate to plate. Try measuring in the forward direction.

At any rate, that's a far cry from under 20 Watt. A quad of 807s would give you around 70 Watt but the power transformer is wound for 450V (as a doubler).

Ironically, while it's overkill power wise the PP impedance is about right for the pair of 6CW5 I mentioned and the PT might fit too (non doubled).

Another possibility is quad 6LR8, again with the PT undoubled, for, I would guess, about 25 Watt. They're about 6 bucks each but the advantage there is you get a mu 58 triode (sort of like half a 12AT7) with them which might be enough (4) for the front end.

Last edited by Flipperhome on Apr Wed 13, 2016 1:18 am, edited 1 time in total.

I have a big box of 6BQ6/EL84 audio power tubes I bought at auction. Let me know if you are interested. I will test and ship you a pair for shipping cost. (about $2.50)

Thank You for the offer, that is very generous, but the impedance of this transformer is too low to make good use of them.

35Z5 wrote:

Yee Hawww...

I'd be riggin' that that Bogen to run a couple 6GT5, those have same basing as 7868...

Tom

Thank You for the suggestion. I spent the night digging through data sheets and drawing load lines. The 6GT5 looks like a perfect match for this output transformer and power transformer (minus voltage doubler). The fact that they have the same basing as the original tubes is pretty slick too.

I plan to "breadboard" this in the Bogen chassis, then after finalizing the design I'll build a chassis to fit the cab and move everything over.